Synthesis of BN-fused polycyclic aromatics via tandem intramolecular electrophilic arene borylation

Article abstract of DOI:10.1021/ja208950c

A tandem intramolecular electrophilic arene borylation reaction has been developed for the synthesis of BN-fused polycyclic aromatic compounds such as 4baza-12b-boradibenzo[g,p]chrysene (A) and 8b,11b-diaza-19b,22b- diborahexabenzo[a,c,fg,j,l,op]tetracene

Full text of DOI:10.1021/ja208950c

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pubs.acs.org/JACS
Synthesis of BN-Fused Polycyclic Aromatics via Tandem
Intramolecular Electrophilic Arene Borylation
Takuji Hatakeyama,*,†,‡ Sigma Hashimoto,† Shu Seki,*,§ and Masaharu Nakamura*,†
†International Research Center for Elements Science, Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan
‡PRESTO, Japan Science and Technology Agency, and §Department of Applied Chemistry, Graduate School of Engineering,
Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan
S Supporting Information
b
Scheme 1. Tandem Intramolecular Electrophilic Arene
Borylation To Synthesize BN-Fused PAHs
ABSTRACT: A tandem intramolecular electrophilic arene
borylation reaction has been developed for the synthesis
of BN-fused polycyclic aromatic compounds such as 4b-
aza-12b-boradibenzo[g,p]chrysene (A) and 8b,11b-diaza-
19b,22b-diborahexabenzo[a,c,fg,j,l,op]tetracene. These com-
pounds adopt a twisted conformation, which results in a tight
and offset face-to-face stacking array in the solid state. Time-
resolved microwave conductivity measurements prove that
the intrinsic hole mobility of A is comparable to that of
rubrene, one of the most commonly used organic semicon-
ductors, indicating that BN-substituted PAHs are potential
candidates for organic electronic materials.
from bis(biphenyl-2-yl)amine12 1, was treated with a variety of
Lewis acids and base additives in o-dichlorobenzene at 150 °C for
12 h. During the initial screening of Lewis acids such as AlCl3
(entry 1), AlBr3, GaCl3, and Zn(OTf)2, the target compound A
was not obtained, and the starting amine 1 was recovered. After
extensive screening, we found that A could be successfully
synthesized (yield: 30%) when using a mixture of AlCl3 (4 equiv)
and NEtiPr2 (1.5 equiv). However, reaction with 0.5, 1.0, and 2.0
equiv of NEtiPr2 did not give the desired product (entries 2ꢀ5).
Screening of the base additives (entries 6ꢀ11) indicated that the
optimum product yield (67%) was achieved when using 1.5 equiv
of 2,2,6,6-tetramethylpiperidine (TMP) (entry 6). Thus, we
concluded that the AlCl3/TMP stoichiometry plays an important
role in enhancing the product yield (entries 12ꢀ16). The sluggish
conversion in the presence of low and high concentrations of the
base was possibly due to the decomposition of the product and
the deactivation of AlCl3, respectively. The other Lewis acids
considered were not as effective as AlCl3 (entries 17ꢀ21). It is
olycyclic aromatic hydrocarbons (PAHs) are an important
P
class of compounds used to produce organic electronics, dyes,
sensors, and liquid crystal displays.1 Replacement of the CꢀC
units in PAHs by isoelectronic BꢀN units affords novel hetero-π-
conjugated molecules that are structurally similar to their all-
carbon analogues but show dramatically different optical and
electronic properties because of the local dipole moment or
polarized frontier orbitals.2 Following the pioneering work of
Dewar,3 intensive efforts have been devoted to the synthesis of
BN-substituted aromatics. Piers,4 Ashe,5 and Liu6 have carried
out extensive research in this direction in recent years.7 However,
because of the lack of a suitable methodology, construction of
polycyclic frameworks with BꢀN units requires multiple steps;4
hence, BN-substituted PAHs have not yet been studied in detail.
We envisioned a tandem electrophilic arene borylation8 as an
efficient means of constructing extended π-conjugated frame-
works with BN ring fusion (Scheme 1). Molecules with such
frameworks are not only attractive functional materials but also
potential starting compounds for the controlled synthesis of BN-
embedded nanocarbons9 by surface-assisted coupling10 or am-
plification sheet growth.11 Since borylation under conventional
conditions did not afford the target compounds, we carefully
screened the reaction conditions and identified that a combination
of a Lewis acid and a Brønsted base in the appropriate ratio
efficiently promotes the tandem electrophilic arene borylation.
We report herein the synthesis and physical properties of 4b-aza-
12b-boradibenzo[g,p]chrysene (A, n = 0) and 8b,11b-diaza-19b,
22b-diborahexabenzo[a,c,fg,j,l,op]tetracene(B, n=1) todemonstrate
potential applications of proposed method and BN-substituted PAHs
in material science.
noteworthy that boron sources other than BCl3, such as BF3 Et2O,
3
BBr3, and B(OMe)3, did not afford the target compound A even
under the optimized conditions.
The structure of A has been determined by X-ray crystal-
lography (Figure 1a). The BꢀN bond length (1.426(3) Å) is
shorter than in typical BN aromatics (1.45ꢀ1.47 Å),3ꢀ5 thus
confirming the double bond character.13 On the other hand, the
BꢀC1, BꢀC2, NꢀC3, and NꢀC4 lengths in A are 1.535(3),
1.534(3), 1.442(3), and 1.448(3) Å, respectively, indicating that
these are single bonds. The aforementioned observations reveal
the low aromaticity of BNC4 rings and are consistent with the
results of nucleus-independent chemical shift (NICS) analysis
(vide infra). Because of steric repulsion between the hydrogen
atoms at the ortho position and the heteroatoms (dihedral angle
C5ꢀC7ꢀC6ꢀC8, 38.87°), A adopts a twisted conformation and
Table 1 summarizes the optimization of the tandem electro-
Received: September 22, 2011
Published: October 25, 2011
philic arene borylation. Dichloroboraneamine 10, prepared in situ
r
2011 American Chemical Society
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Table 1. Screening of Lewis Acids and Brønsted Bases
Lewis acid
(equiv)
yieldb
entrya
additive (equiv)
(%) of A
1
AlCl3 (4.0)
AlCl3 (4.0)
AlCl3 (4.0)
AlCl3 (4.0)
AlCl3 (4.0)
AlCl3 (4.0)
AlCl3 (4.0)
AlCl3 (4.0)
AlCl3 (4.0)
AlCl3 (4.0)
AlCl3 (4.0)
AlCl3 (4.0)
AlCl3 (4.0)
AlCl3 (3.0)
AlCl3 (5.0)
AlCl3 (6.0)
AlBr3 (4.0)
AlBr3 (4.0)
GaCl3 (4.0)
GaCl3 (4.0)
none
NEtiPr2 (0.5)
NEtiPr2 (1.0)
NEtiPr2 (1.5)
0
2
0
3
0
4
30
5
6c
NEtiPr2 (2.0)
0
2,2,6,6-tetramethylpiperidine (1.5)
1,2,2,6,6-pentamethylpiperidine (1.5)
2,4,6-collidine (1.5)
67d
27
27
<1
15
16
0
7
8
9
Proton Sponge (1.5)
10
11
12
13
14
15
16
17
18
19
20
21
NCy2Me (1.5)
iBu3N (1.5)
2,2,6,6-tetramethylpiperidine (1.0)
2,2,6,6-tetramethylpiperidine (2.0)
2,2,6,6-tetramethylpiperidine (1.5)
2,2,6,6-tetramethylpiperidine (1.5)
2,2,6,6-tetramethylpiperidine (2.25)
NEtiPr2 (1.5)
42
0
34
28
30
36
38
6
2,2,6,6-tetramethylpiperidine (1.5)
NEtiPr2 (1.5)
2,2,6,6-tetramethylpiperidine (1.5)
Zn(OTf)2 (4.0) NEtiPr2 (1.5)
0
a Reactions were carried out on a 0.6 mmol scale. b The yield was
determined by 1H NMR using 1,1,2,2-tetrachloroethane as internal stan-
dard. c The reaction was carried out on a 15 mmol scale. d Isolated yield.
Figure 1. ORTEP drawing and packing structure of A (a,b) and
dibenzo[g,p]chrysene (c,d). Thermal ellipsoids are shown at 50%
probability; hydrogen atoms are omitted for clarity. (P)-enantiomer,
blue; (M)-enantiomer, pink.
hence has a unique packing structure (Figure 1b). That is, the
molecules are arranged in an offset face-to-face stacking array
with a πꢀπ distance of 3.3ꢀ3.6 Å; in this arrangement, the local
dipole moments of the BꢀN bonds also offset each other. Each
array includes an enantiomer with a left- or right-handed helical
structure (M-helix (shown in pink) or P-helix (shown in blue),
respectively) with CHꢀπ interaction (3.0ꢀ3.4 Å). To under-
stand the isoelectronic relationship between BꢀN and CꢀC units,
the structure of the isoelectronic carbon analogue, dibenzo[g,p]
chrysene, was determined by X-ray crystallography (Figure 1c,d).
The central C1ꢀC2 bond is highly olefinic (1.3882(16) Å) and
much shorter than the four radial CꢀC bonds (1.4608(19)ꢀ
1.4657(18) Å), indicating small degree of bond alternation and
low aromaticity of the central rings, as observed in A. The molecular
arrangement in this analogue is the same as that in A, and
surprisingly, the lattice constants, too, are nearly equal to those of
A. Further, BN substitution does not cause any notable change in the
melting point (A, 227 °C; dibenzo[g,p]chrysene, 229 °C) but
significantly improves the organic solubility (solubility in Et2O and
AcOEt: A, 4.4 and 7.7 mg/mL; dibenzo[g,p]chrysene, 2.5 and
5.0 mg/mL), probably owing to its dipole moment.
one of the most popular organic semiconductors.14d Interestingly,
the hole mobility of dibenzo[g,p]chrysene is only one-tenth that of
A (0.007 cm2 Vꢀ1 sꢀ1). The transient conductivity is not quenched
even in a SF6 environment, suggesting that the major charge carriers
in A are positive charges (holes). The pseudo-first-order decay
kinetics (Figure 2a) also indicate that the mobile holes are trapped
rapidly by impurities/structural defects at interfaces, rather than the
bulk recombination with negative charges via second-order reactions
in the polycrystalline phases.
To gain a deeper insight into the hole mobility, the electronic
coupling V (meV) between neighboring molecules is calculated
from the X-ray crystal structures of A and dibenzo[g,p]chrysene
(Figure 3).15,16 Although the molecular packing is virtually the
same in these compounds, the electronic coupling varies sig-
nificantly. The V values along the a-axis in A (6.1 and 2.4 meV,
Figure 3a) are 10 times larger than the corresponding values in
dibenzo[g,p]chrysene (0.6 and 0.3 meV, Figure 3c), while those
along the b-andc-axes are comparable to each other (Figures 3aꢀd).
We suppose that the BN-substitution-induced partial localization of
the frontier orbitals (vide infra) strengthens the electronic coupling
Time-resolved microwave conductivity (TRMC) mea-
surements14 confirm that A has high intrinsic hole mobility
(0.07 cm2 Vꢀ1 sꢀ1), rivaling that of rubrene (0.05 cm2 Vꢀ1 sꢀ1),
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Figure 2. (a) Conductivity transients monitored for the polycrystalline
phase of A in Ar (blue) and SF6 (violet) environments. Identical traces
were observed for both atmospheres. The kinetic trace for transient
conductivity was also observed for dibenzo[g,p]chrysene (red). Excita-
tion was carried out at 355 nm using 13 mJ cmꢀ2 pulses for all transients.
Dependence of ϕ on the applied electric field strength between inter-
digitated Au electrodeswitha 5-μm gap. Thevalues of ϕ were determined
by photocurrent integration under illumination of a thin film of A
(120 nm thick) and dibenzo[g,p]chrysene (80 nm thick) at 355 nm,
22 mJ cmꢀ2. The photocurrent under 355-nm illumination was also
traced using identical samples; the IꢀV traces are illustrated in (b). Blue
dashed and red dotted lines are the traces of A (dark and illuminated,
respectively), and turquoise solid and orange dot-dashed lines are the
traces of dibenzo[g,p]chrysene (dark and illuminated, respectively).
Figure 3. Electronic coupling V (meV) between neighboring molecules
in the X-ray crystal structures of A (a,b) and dibenzo[g,p]chrysene (c,d).
along the a-axis in A, leading to improvement of the total hole
mobility. The reorganization energies of Aand dibenzo[g,p]chrysene
are 0.21541 and 0.20413 eV, respectively.14,15 Such a small difference
is unlikely to be a dominant factor affecting the difference in
the hole mobilities of these compounds.
In cyclic voltammetry (CV) experiments, reversible oxidation
and reduction waves with peak potentials at +0.76 V (+0.89 V)
and ꢀ2.77 V (ꢀ2.65 V) vs Fc/Fc+, respectively,17 are observed
for A (dibenzo[g,p]chrysene). BN substitution leads to a nega-
tive shift in the redox potential, which can partially account for
the improved hole mobility.
Figure 4. NICS(1) values of A and dibenzo[g,p]chrysene, and the
KohnꢀSham LUMO and HOMO of A at the B3LYP/6-31G* level.
Scheme 2. Two-Step Synthesis of B
The aromaticities of the six-membered rings in A and dibenzo-
[g,p]chrysene are evaluated by NICS calculations (Figure 4).18
Interestingly, the BNC4 rings in A have low aromaticity, as
suggested by their NICS(1) value of ꢀ2.9,19 whereas the
corresponding C6 rings in dibenzo[g,p]chrysene have moderate
aromaticity (ꢀ6.5). On the other hand, BN substitution does not
significantly affect the aromaticity of the surrounding C6 rings in
A (cf. benzene:20 ꢀ11.2). Despite the low aromaticity of the
central BNC4 rings, the π-conjugation in A is extended over the
entire molecule and reasonable polarization16 is observed, as indi-
cated by molecular orbital calculations. It should be noted that similar
trends were observed in extended π-conjugated molecule B.21
Based on the present synthetic strategy, we state that an
extended π-conjugated framework with two fused BN rings can
be synthesized in two steps from commercially available sources
(Scheme 2). N,N0-Bis(biphenyl-2-yl)biphenyl-2,6-diamine (2),
which was prepared from 2,6-dichlorobiphenyl and 2-aminobiphenyl
in 87% yield, was used for the synthesis of 8b,11b-diaza-19b,22b-
diborahexabenzo[a,c,fg,j,l,op]tetracene (B).22 Introduction of boron
substituents in 2 and subsequent tandem electrophilic arene boryla-
tion afforded B in 32% yield.
B is moderately soluble in organic solvents such as chlorobenzene
(6.7 mg/mL) and 1,2-dichlorobenzene (8.6 mg/mL), probably
owing to its flexible molecular framework and dipole moment.
These advantageous physical properties of B make it suitable for
use in organic electronics.
In summary, we have developed a tandem intramolecular electro-
philic arene borylation to synthesize BN-fused polycyclic aromatic
compounds that adopt twisted geometries. TRMC measurements
confirm that substitution of the CꢀC units in dibenzo[g,p]chrysene
with isoelectronic BꢀN units dramatically enhances hole mobility,
owing to the strong electronic coupling between neighboring
molecules in the solid state. Our strategy is simple and practical,
and it can be employed for the extension of π-conjugated frame-
works, thus forming the basis for pioneering research in material
science and spurring the development of bottom-up approaches
toward BN-embedded nanocarbons.
The results of CV experiments indicate that B shows an
irreversible oxidation wave and a reversible reduction wave with
peak potentials at +0.10 and ꢀ1.57 V, respectively (vs Fc/Fc+).
While the electrochemical HOMOꢀLUMO gap (1.67 eV) in B is
smaller than that in pentacene (2.09 eV),23 no obvious decom-
position was observed, even at the melting point (358 °C) under
atmospheric conditions. Despite its polycyclic aromatic structure,
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’ AUTHOR INFORMATION
Corresponding Author
hatake@scl.kyoto-u.ac.jp; seki@chem.eng.osaka-u.ac.jp; masaharu@
scl.kyoto-u.ac.jp
’ ACKNOWLEDGMENT
This research was supported by the Japan Society for the
Promotion of Science (JSPS) through the “Funding Program for
Next Generation World-Leading Researchers (NEXT Program)”,
initiated by the Council for Science and Technology Policy
(CSTP). The study was also supported by a Grant-in-Aid for
Scientific Research on Innovative Areas “Reaction Integration”
(No. 2105, 22106524) and a Grant-in-Aid for Young Scientists
(23685020) from MEXT and JSPS, and Asahi Glass Foundation.
We are grateful to Professors Hikaru Takaya and Takahiro Sasamori
(Kyoto University) for their guidance in the X-ray crystallography
experiments. We also thank Yasujiro Murata (Kyoto University) for
permitting us to use the cyclic voltammeter at the university.
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S.-Y. J. Am. Chem. Soc. 2011, 133, 11508.
(21) NICS(1) values for the four BNC4 rings in B are ꢀ2.2 to ꢀ2.6,
while those for the six C6 rings are ꢀ10.1 to ꢀ11.2. See the Supporting
Information for details.
(7) Recent examples for aromatic compounds with boron and nitrogen
substituents: (a) Pro~n, A.; Zhou, G.; Norouzi-Arasi, H.; Baumgarten, M.;
M€ullen, K. Org. Lett. 2009, 11, 3550. (b) Sachdev, H.; Zahn, N.; Huch, V. Z.
Anorg. Allg. Chem. 2009, 635, 2112. (c) Agou, T.; Sekine, M.; Kobayashi, J.;
Kawashima, T. Chem.—Eur. J. 2009, 15, 5056. (d) Wakamiya, A.; Mori, K.;
Araki, T.; Yamaguchi, S. J. Am. Chem. Soc. 2009, 131, 10850. (e) Lorbach,
(22) Isoelectronic carbon analogue: Mochida, K.; Kawasumi, K.;
Segawa, Y.; Itami, K. J. Am. Chem. Soc. 2011, 133, 10716.
(23) Eox = 0.22 V, Ered = ꢀ1.87 V: Sakamoto, Y.; Suzuki, T.;
Kobayashi, M.; Gao, Y.; Fukai, Y.; Inoue, Y.; Sato, F.; Tokito, S. J. Am.
Chem. Soc. 2004, 126, 8138.
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